Magnetically coupled signal isolator using a faraday shielded MR or GMR receiving element

ABSTRACT

An isolator having a driver circuit which responsive to an input signal drives appropriate signals into one or more coils which are magnetically coupled to one or more corresponding MR or GMR elements whose resistance is variable in response to the magnetic field applied by the coil(s), and an output circuit that converts the resistance changes to an output signal corresponding to the input signal. A Faraday shield is interposed between the coil(s) and the MR or GMR elements. Common mode transients applied to the driver are capacitively coupled from the coil(s) into the Faraday shield and therethrough to ground, instead of into the MR elements. A second Faraday shield may be disposed in spaced relationship with the first Faraday shield and referenced to the potential of the MR elements for even greater common mode rejection. The entire structure may be formed monolithically as an integrated circuit on a single substrate, for low cost, small size, and low power consumption. With proper driver and receiver circuits, the isolator may transmit either analog or digital signals.

RELATED APPLICATION

This application claims priority to Provisional Application Serial No.60/063,221, filed Oct. 23, 1997, and entitled: “Magnetically CoupledSignal Isolator Using a Faraday Shielded MR or GMR Receiving Element”,the entire contents of which are incorporated herein by reference.

This application is a continuation of application Ser. No. 09/118,032,filed Jul. 17, 1998, now U.S Pat. No. 6,054,780, entitled MAGNETICALLYCOUPLED SIGNAL ISOLATOR USING A FARADAY SHIELDED MR OR GMR RECEIVINGELEMENT, and now pending, which claims the benefit of provisionalapplication Ser. No. 60/063,221 filed Oct. 23, 1997.

FIELD OF THE INVENTION

This invention relates to the field of circuitry for isolating analogand digital electronic signals, such as to provide galvanic isolationbetween signal sources in a process control system and amplifiers ormicrocontrollers receiving signals from those sources, or betweenmicrocontrollers and other signal sources and transducers or otherdevices using those signals.

BACKGROUND OF THE INVENTION

In a variety of environments, such as in process control systems, analogor digital signals must be transmitted between diverse sources andcircuitry using those signals, while maintaining electrical (i.e.,galvanic) isolation between the sources and the using circuitry.Isolation may be needed, for example, between analog sensors andamplifiers or other circuits which process their output, or betweenmicrocontrollers, on the one hand, and sensors or transducers whichgenerate or use microcontroller input or output signals, on the otherhand. Electrical isolation is intended, inter alia, to preventextraneous transient signals, including common-mode transients, frominadvertently being processed as status or control information, or toprotect equipment from shock hazards or to permit the equipment on eachside of an isolation barrier to be operated at a different supplyvoltage, among other known objectives. One well-known method forachieving such isolation is to use optical isolators that convert inputelectrical signals to light levels or pulses generated by light emittingdiodes (LEDs), and then to receive and convert the light signals backinto electrical signals. Optical isolators present certain limitations,however: among other limitations, they are rather non-linear and notsuitable for accurate linear applications, they require significantspace on a card or circuit board, they draw a large current, they do notoperate well at high frequencies, and they are very inefficient. Theyalso provide somewhat limited levels of isolation. To achieve greaterisolation, opto-electronic isolators have been made with some attemptsat providing an electrostatic shield between the optical transmitter andthe optical receiver. However, a conductive shield which provides asignificant degree of isolation is not sufficiently transparent for usein this application.

One isolation amplifier avoiding the use of such optical couplers in adigital signaling environment is described in U.S. Pat. No. 4,748,419 toSomerville. In that patent, an input data signal is differentiated tocreate a pair of differential signals that are each transmitted acrosshigh voltage capacitors to create differentiated spike signals for thedifferential input pair. Circuitry on the other side of the capacitivebarrier has a differential amplifier, a pair of converters for comparingthe amplified signal against high and low thresholds, and a set/resetflip-flop to restore the spikes created by the capacitors into a logicsignal. In such a capacitively-coupled device, however, during a commonmode transient event, the capacitors couple high, common-mode energyinto the receiving circuit. As the rate of voltage change increases inthat common-mode event, the current injected into the receiverincreases. This current potentially can damage the receiving circuit andcan trigger a faulty detection. Such capacitively coupled circuitry thuscouples signals that should be rejected. The patent also mentions,without elaboration, that a transformer with a short R/L time constantcan provide an isolation barrier, but such a differential approach isnonetheless undesirable because any mismatch in the non-magnetic (i.e.,capacitive) coupling of the windings would cause a common-mode signal toappear as a difference signal.

Another logic isolator which avoids use of optical coupling is shown incommonly-owned, unpublished U.S. patent application Ser. No. 08/805,075,filed Feb. 21, 1997, in the name of Geoffrey T. Haigh, titled “LogicIsolator with High Transient Immunity,” incorporated by referenceherein. This logic isolator exhibits high transient immunity, forisolating digital logic signals, such as signals between equipment on afield side (i.e., interfacing with physical elements which measure orcontrol processes) and microcontrollers on a system control side, usefulin, for example, a process control system. In one aspect, the logicisolator has an input circuit that receives a digital input signal, withedge detection circuitry that detects rising and falling edges of thatinput signal. The logic circuit provides an output signal indicative ofthose rising and falling edges to a transformer assembly which serves asan isolation barrier. The transformer assembly replicates the signal itreceives and provides it to an output circuit, while shunting capacitivecommon-mode transient currents to ground. The output circuit convertsthe signal from the transformer back into a digital logic signal withrising and falling edges as in the digital input signal, those slightlydelayed therefrom. The transformer assembly preferably includes alink-coupled transformer that has a first core with a first winding, asecond core with a second winding, and a grounded link wire that extendsfrom the first core to the second core for grounding capacitively-linkedcommon-mode transients. Alternatively, a shielded transformer, with agrounded double- or single-shield between primary and secondarywindings, could be used.

In certain described embodiments, the input circuit converts the risingand falling edges in the digital input signal to positive and negativepulses using tri-level logic, and the output circuit converts thesepulses back into rising and falling edges. The input circuit preferablyalso includes a pulse generator for providing pulses, referred to asrefresh pulses, with a high frequency and with a pulse width that is thesame as the width of the pulses created in response to detection of arising edge or a falling edge. The refresh pulses are logically combinedwith the input signal to provide an interrogating functionality thatallows the isolator to determine the DC- or steady- state of the inputsignal; therefore, the isolator can recover quickly in case of a powerspike or dropout, and also can quickly determine the state if an edge ismissed. The isolator has circuitry that inhibits the first refresh pulseafter an edge to prevent a double-wide pulse being transmitted;consequently, the isolator can interrogate the state of the input signala time t after an event, with T<t<2T—i.e., no later than 10 μs if therefresh pulses have a period of 5 μs.

A need, however, exists for an isolation technology which is useful forboth analog and digital signals. A further need exists for isolatorswhich are manufacturable with still lower cost than the aforementionedtypes of isolators, operating at lower power and manufacturable in verysmall size. Even more specifically, a need exists for isolatorsemploying magneto-resistive (MR) and giant magneto-resistive (GMR)effects and which are very fast in operation (being useful in thenanosecond domain).

SUMMARY OF THE INVENTION

The present invention addresses these needs by providing an isolatorwherein an input signal is coupled from an input node to amagnetic-field generator and the magnetic field generated thereby iscoupled to one or more corresponding MR or GMR elements whose resistanceis variable in response to the magnetic field, with an output circuitthat converts the resistance changes to an output signal correspondingto the input signal. A Faraday shield is interposed between the coil(s)and the MR or GMR elements. (Hereafter, the term MR will be usedgenerically, except where otherwise noted from context, to include bothmagneto-resistive and giant magneto-resistive elements.) The inputsignal is referenced to a first ground, or reference potential, and theoutput signal is referenced to a second ground, or reference potential.Common mode transients are capacitively coupled from the coil(s) intothe Faraday shield and therethrough to the second ground, instead ofinto the MR elements. The magnetic-field generator may include one ormore coils and a driving circuit coupled between the input node and thecoil or coils.

According to an aspect of the invention, two Faraday shields may bedisposed in spaced relationship between the coil(s) and the MR elements.In such an arrangement, a first Faraday shield is at the first referencepotential and the second Faraday shield is at the second referencepotential.

According to certain aspects of the invention, the MR elements comprisefour magnetically-sensitive resistors elements arranged in a bridge,with diagonally opposing pairs of such resistors receiving the magneticfield from each of first and second input coils, respectively. Theoutput nodes of the bridge are connected to differential inputs of adifferential receiver.

In one aspect, an isolator according to the invention may bemonolithically fabricated. Either one die or two may be used. With twodie, the driver circuitry may, for example, be formed on a firstsubstrate and the coil(s), MR element(s) and receiver may be formed on asecond substrate.

An embodiment is shown with a complete isolator formed monolithically ona single die.

With appropriate driver and receiver circuits, the isolator is usefulfor either analog signals or digital signals. Exemplary driver andreceiver circuits for each type of signal are shown.

The foregoing and other features, advantages and alternative embodimentsof the invention, will be more readily understood and become apparentfrom the detail description which follows, which should be read inconjunction with the accompanying drawings and claims. The detaileddescription will be understood to be exemplary only and is not intendedto be limited of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing,

FIG. 1 is a schematic circuit diagram for a first exemplaryimplementation of an isolator according to the present invention;

FIG. 2 is a waveform diagram illustrating operation of the circuit ofFIG. 1 as a digital signal isolator;

FIG. 3 is a logic diagram for a driver circuit suitable for use in thedigital isolator of FIG. 2;

FIG. 4 is a waveform diagram illustrating operation of the circuit ofFIG. 3;

FIG. 5 is a diagrammatic, exploded view showing conceptually how anisolator according to the present invention may be fabricated usingintegrated circuit manufacturing techniques;

FIG. 6 illustrates in a simplified side view, schematically, the layersof materials that form monolithically the elements of an isolatoraccording to the present invention;

FIG. 7 is a simplified diagrammatic, isometric view, partially in crosssection, showing how a complete isolator according to the presentinvention, with an input driver circuit, may be fabricatedmonolithically on a single substrate; and

FIG. 8 is a simplied top view of a portion of an exemplary isolatoraccording to the invention, showing the spatial relationship among themagnetic-field generating coil(s), a Faraday shield andmagnetoresisitive sensor elements.

DETAILED DESCRIPTION

An exemplary implementation of an MR isolator 10 in accordance with thepresent invention is illustrated schematically in FIG. 1. An inputvoltage is supplied at port 12 to a magnetic field generator 13,comprising an input driver 14 and one or more coils L1, L2. Driver 14supplies output signals DRIVE A and DRIVE B on lines 16 and 18,respectively, to espective coils L1 and L2. Each of coils L1 and L2generates a magnetic field which is sensed by a bridge 20 formed by MRelements 22, 24, 26 and 28. Elements 22 and 24 are connected in seriesacross the supply rails as are elements 26 and 28. The bridge provides adifferential output across nodes 32 and 34 at the respective junctionsbetween resistors 22 and 24 on the one hand, and 26 and 28 on the other.Node 32 supplies a first signal RCVDC on line 36 to a non-invertinginput of a differential receiver 40 and node 34 supplies a secondreceived signal RCVDD on line 38 to the inverting input of the receiver40. The output of the isolator appears on line 42 at the output ofreceiver 40. Since galvanic isolation of the output from the input is aprincipal objective, the input is referenced to a first ground, GND1,and the ouput is referenced to a second ground, GND2. A Faraday shield50 connected to ground GND2, is interposed between the coils L1, L2, onthe one hand, and bridge 20, on the other. Faraday shield 50 provideselectrostatic isolation between the coils and the MR bridge whileallowing the magnetic fields generated by the coils to pass through tothe MR elements of the bridge. Specifically, the field generated by coilL1 passes through elements 22 and 28 while the field generated by coilL2 passes through the elements 24 and 26.

For use as an analog signal isolator, the driver 14 may typicallyprovide signals DRIVEA and DRIVEB as a pair of differential outputsignals. Some wave-shaping or signal conditioning may be applied indriver 14 or in receiver 40, as appropriate to the applications.

For use as a digital signal isolator, operation of isolator 10 circuitmay be understood with reference to the waveforms of FIG. 2. In FIG. 2,it is assumed that the input signal is a voltage having a waveformrepresenting a logic signal illustrated at 60. Prior to time T₁, signal60 is low. At time T₁, the input goes from a low to a high value anddriver 14 presents a pulse 72 of a short, predetermined width andamplitude in the signal DRIVEA. At the falling edge of the input signal,at time T₂, a comparable pulse 74 is generated by driver 14 in thesignal DRIVEB. The corresponding received signals detected at nodes 32and 34 are shown in the waveforms for the signals RCVDC and RCVDD. Thereceiver 40 is a comparator with a slight amount of hysteresis, whichessentially operates as a bistable element. The “pulse” 76 generated inthe RCVDC signal by DRIVEA pulse 72 sets the output signal high, and thepulse 78 generated in the RCVDD signal generated by the DRIVEB pulse 74resets the output signal to a low level. Thus, the output signalrecreates the input signal faithfully.

The amount of hysteresis employed in receiver 40 preferably is selectedto assure a high reliability of set and reset operation of the receiverwhile obtaining as much insensitivity to noise as possible.

While numerous circuits may be employed for driver 14 in the digitalsignal processing mode, an exemplary circuit 14A is shown in FIG. 3. Theinput signal applied to port 12 is supplied to an odd number ofinverters 82-1 through 82-N (three inverters may suffice), as well as toone input of each of NOR-gate 84 and AND gate 86, as well as to pulsegenerator 88. (Pulse generator 88 is optional and its use is describedadequately in the aforementioned patent application of Geoffrey Haigh.)A second input of each of gates 84 and 86 is supplied from the output ofthe inverter string 82-1 through 82-N. The output of NOR-gate 84supplies the DRIVEA signal on line 16 to coil LI and the output of ANDGATE 86 supplies the DRIVEB signal on line 18 to coil L2.

The operation of the circuit of FIG. 3 is now explained with referenceto the waveforms of FIG. 4. The input signal again is assumed to be alogic signal which is high between times T₁ and T₂. The delayed andinverted state of the input signal which appears at node 92, termed D-IINPUT, thus is a copy of the input signal, inverted and delayed by thepropagation delay of the inverter chain 82-1 through 82-N, which delayis labeled in the drawing as Δt. It is assumed that Δt is much smallerthan the interval from T₁ through T₂. For example, Δt is typically justa few nanoseconds. The output from NOR-gate 84 consequently is highexcept during interval from T₂ to T₂+Δt; and the output of the AND gate86, the DRIVEB signal, is high except in the interval from T₁ to T₁+Δt.

A diagrammatic illustration, as shown in FIG. 5, is useful to illustrateconceptually how such an isolator may be fabricated monolithically. Suchfabrication may occur with the driver on a first substrate, SUB 1), andwith the coils, Faraday shield, MR sensor and receiver on a secondsubstrate, SUB2, or with the entire apparatus on a single substrate(ie., where SUB1 and SUB2 are the same substrate), as more fullyexplained below.

Without indicating any patterning, FIG. 6 shows a schematic side view ofthe layers of materials that form monolithically the coils, Faradayshield, sensor and receiver of FIG. 5. The resistive sensors 110 areformed on or in a semiconductor substrate 112 along with the receivercircuitry indicated generally in area 114. A thin layer of oxide 116 isthen formed over the substrate. This is followed by a metallizationlayer 117 which connects to the substrate (i.e., the input's ground) andwhich provides the Faraday shield; appropriate positioning and areaconsiderations are discussed below). A thick oxide layer 118 is appliedover the metallization 117. On top of the thick oxide layer 118 there isformed a metallization layer 120 which is patterned to form coil L1 andL2 in appropriate geometric relationship and placement over sensorelements 110.

Turning to FIG. 7 there is generally illustrated a single substrateembodiment containing the entire isolator. The driver circuitry 14 iselectrically isolated from the sensors 20 and receiving circuitry 40 bybuilding the entire isolator structure on an oxide layer formed over thesubstrate 112 and then surrounding the driver and/or sensors andreceiver by one or more dielectric isolation zones, also calledtrenches, 132 which are filled during fabrication with an oxide or otherdielectric material. To avoid obfuscation, the coils are not drawn butare represented operatively by the dashed line M, representing amagnetic linking.

Using a trench-isolated ICE manufacturing process, approximately onekilovolt of isolation is provided per micrometer of oxide (or nitride orsimilar dielectric) thickness. With a base oxide layer and trenchesthree micrometers thick, approximately three kilovolts of isolation willbe achieved. This is satisfactory for a large number of typicalapplications and it can be increased for other applications.

A top view, in diagrammatic form, of an exemplary geometry for a singlecoil-shield sensor arrangement is shown in FIG. 8. The Faraday shield FSwhich is interposed between the coil L and the sensor S_(MR) is a highlyconductive surface, such as a metal, which does not form a closed loopof high permeability “short circuiting” the magnetic field. Thus, ametal patch area over the sensor is sufficient, where the surface areaof the metal patch does not span the whole coil. The orientation of thesensor resistors relative to the coil may be significant. MR and GMRresistors generally change their resistance in response to the appliedmagnetic field when the magnetic field lines are oriented longitudinallywith the resistor. Thus, in the illustration, the MR resistors of sensorS_(MR) are shown oriented horizontally while the coil windings aresubstantially vertical where they span the sensor.

A typical opto-isolator draws a steady current of about 3-15 mA for highspeed digital operation, using a supply voltage in the three to fivevolt range. By contrast, the exemplary apparatus of FIG. 2 et seq. drawsvery little current except during the drive pulses. With a 50 MHz clockspeed and a pulse width, Δt, of one nanosecond, if the current drawnduring the pulse is 10 mA, the average current is only 09.5 mA. At lowclock speeds or data rates such as a 50 Hz rate as might be used inmedical electronics, for example, the drive pulses consume an averagecurrent of only about 0.5 microamps. Even considering the currentrequired for operating the receiver and quiescent driver circuits, theentire apparatus may be operated on only about 10-12 microamps.Additionally, magnetoresistive elements are very fast to respond tochanges in magnetic field, reacting in the nanosecond domain. As aresult, an isolator in accordance with the invention should be muchfaster (e.g., ten times faster) than an opto-isolator.

For GMR elements, the change in resistance over the range of magneticfield from a zero field to a saturation field is only about 1-4 percent.When a five volt power supply is used, this means the GMR elementsproduce only about a 50-200 millivolt signal swing. The capacitivecoupling between the coils and the GMR elements may be about 0.1-1 pFwithout the Faraday shield. If a transient common mode voltage isimposed on driver 14, it is capacitively coupled from the output ofdrive 14 into Faraday shield 50, and the capacitive current is coupledto ground.

Numerous design considerations must be taken into account whenassembling such an isolator, in addition to those already discussed.These are easily within the skill of circuit design and semiconductorengineers. For example, the MR elements must be placed relative to themagnetic field provided by the coils so as, preferably, to have themagnetic field direction coincide substantially with the sensors'lengthwise, most-sensitive, direction. The MR elements will thusgenerate the greatest output for a given magnetic field if they (the MRelements) are all similarly oriented relative to the magnetic field. Auseful arrangement, as depicted in top view in FIG. 8 is to form thecoils as about six substantially rectangular turns of conductor, formingan inductance of about 1 nH, disposing the magneto-resistive elementsunder one side of the rectangle, with their magnetically-sensitivedirection being transverse to the rectangle's side. (With such a smallinductance, it is important that the driver circuit act as a very goodcurrent source.) The Faraday shield should be large enough to span theMR elements but not so large as to have it interfere significantly withthe magnetic field from the coils cutting through the MR elements.

Having thus described the invention and various illustrative embodimentsof the invention, some of its advantages and optional features, it willbe apparent that such embodiments are presented by way of example onlyand not by way of limitation. Those persons skilled in the art willreadily devise alterations and improvements on these embodiments, aswell as additional embodiments, without departing from the spirit andscope of the invention. For example, it will be appreciated thatalthough the MR sensor is shown as a bridge circuit in the illustratedembodiments, a single MR element or two MR elements might be employed,instead, and four elements might be arranged in a manner other than as abridge. Likewise, though two coils are shown as the magnetic fieldgeneration members, one might choose to use just one coil, or some othernumber than two, with appropriate driver circuitry. The driver circuitis not needed in all cases, as the input signal source may be able todrive the coils directly. Alternatively, some other magnetic-fieldgenerating apparatus may be employed. It is impossible to enumerate allof the variations that will quite quickly occur to those in the art.Accordingly, the invention is limited only as defined in the followingclaims and equivalents thereto.

What is claimed is:
 1. A signal isolator comprising: a. an input nodefor receiving an input signal; b. at least one magnetic-field generationmember actuable to generate a magnetic field corresponding to the inputsignal; c. a magnetoresistive sensor positioned to receive and beinfluenced by the magnetic field generated by the at least one magneticfield generation member, such that the sensor supplies at an output nodea signal corresponding to the magnetic field; and d. a Faraday shielddisposed between the at least one magnetic-field generation member andthe sensor, said shield being electrically referenced to a samepotential as the output node.
 2. The isolator of claim 1 wherein themagnetic-field generation member includes at least one coil whichgenerates said magnetic field.
 3. The isolator of claim 2 wherein themagnetic-field generation member further includes a driver circuitcoupled between the input node and the at least one coil and driving theat least one coil.
 4. The isolator of claim 3 wherein the input signalis a digital signal, the at least one coil includes first and secondcoils, and the driver circuit drives the first and second coils withshort pulses of current in response to logic value changes in the inputsignal.
 5. The isolator of any of claims 1-4 further including areceiver circuit coupled to the sensor and supplying an output signal atan output thereof.
 6. The isolator of claim 5 wherein the isolator isformed on a single semiconductor substrate.
 7. The isolator of any ofclaims 1-4 wherein the isolator is formed on a single semiconductorsubstrate.